Polarization vision contains important information about the imaged environment, such as surface shapes, curvature and material properties, which are ignored with traditional imaging systems. Several species of invertebrate, such as cuttlefish, honeybees, desert ants, and others, rely on contrast enhancement using polarized vision, which is a vital survival mechanism in optically scattering media. The unaided human eye perceives visual information in terms of color and brightness, but it cannot effectively sense and utilize polarization. Accordingly, it is desired to develop an imaging system capable of extracting polarization information from the imaged environment in real-time and presenting the polarization information in parallel with the intensity information. The sensory system of the invention integrates imaging, a micro-polarization array and polarization processing at the focal plane to provide such an imaging system.
A survey of the literature reveals that most polarization sensitive imaging systems compute contrast enhancement information. This information is extracted by either temporally sampling two images filtered with two orthogonal polarized filters or integrating two orthogonal polarized filters over two neighboring photo elements. The contrast extraction information is computed either on a DSP/CPU or at the focal plane with translinear circuits. Usual tradeoffs in these systems are reduction of frame rate versus reduction of the spatial resolution in the latter systems. Incorporating pixel pitch matched polarization filters at the focal plane has been achieved using birefringence materials or thin film polarizers. Although these sensory systems are directly inspired from biological systems, they present limited polarization information in scattered media, such as fog, under water imaging and others. In contrast, complete polarization information tends to be far more complex and its computational demands prevent real-time extraction. These complex polarization properties are fully described by the fundamental parameters known as the Stokes parameters. As will be explained in more detail herein, in order to fully determine the Stokes parameters of natural (polychromatic) light, the scene must be sampled with three different polarization filters.
A micro-polarization array with two spatially distributed polarizers has been fabricated and described by Guo et al. as described in an article entitled “Fabrication of high-resolution micropolarizer array,” Optical. Engineering, Vol. 36, No. 8, pp. 2268-2271 (1997). Manipulation of polymer polarization filters in order to create a micro-polarizer array has been described by Faris in U.S. Pat. No. 5,327,285. As will be appreciated by those skilled in the art, one of the main challenges in manipulating a relatively thick polymer polarizing filter is the patterning and etching of the structures within 1 μm accuracy. The thickness of commercially available polymer polarization thin films varies between 10 μm to 20 μm, which creates problems when standard etching techniques employed in the semiconductor industry are used to create micro structures on the order of 10 μm or less.
Traditional imaging systems focus on converting light's intensity and color property into suitable electronic signals. An important property of light, polarization is ignored with these traditional imaging systems. Polarization vision contains information about the imaged environment, such as surface shapes, curvature and material properties. Real-time extraction of polarization properties would further allow synergy with traditional adaptive spatiotemporal image processing techniques for synthetic imaging. However, to date, no camera has enabled computation of the Stokes parameters in real-time using a single camera, for conventionally at least three cameras are needed, each with a different polarization filter, or a single camera sequentially computes the Stokes parameters by sequentially imaging with different filters. The present invention has been designed to provide for such real-time extraction using a specially designed polarimetric sensor including a two layer micro-polarizer array in combination with a photo pixel array, noise suppression circuitry and on-chip processing circuitry for polarimetric computation of the Stokes parameters of a partially polarized (polychromatic) light in real-time.